The invention relates to a process and a device useful therein for hydraulically entangling the fibers in a fiber batt to form a fiber-entangled base sheet for a non-woven fabric. More particularly, it relates to a process utilizing a device having a slot defined therein through which a curtain of liquid is applied against the fiber batt. The curtain is substantially non-diverging and uniform in width throughout the length of the slot when applied under a pressure sufficient to cause fiber entanglement.
It has been known in the past to form loosely interconnected fiber batts by wet, dry, and air lay techniques. To form a permanently bonded, non-woven fabric, the fibers of the batt must be permanently connected together. Generally, two methods have been utilized to hold the individual fibers together, so that they form an integrated web of permanently associated fibers.
In a first method, a fiber web having strength and coherency is produced from the batt by the application of an adhesive or bonding agent at selected points of interconnection of the fibers. The web strength which can be achieved by such a method and the utility of the bonded web obtained depend upon the bonding strength of the agent utilized and its ability to maintain its bonding strength during the normal usage of the fabric manufactured. However, in many applications, the use of an adhesive agent is undesirable, particularly when it is used in amounts sufficient to alone hold the fibers of the web together. This is because of the deleterious effect of adhesive agents on the final characteristics of the fabric manufactured.
A second method, which has been utilized to permanently interconnect the fibers of the batt to form a coherent web, involves mechanically entangling the fibers by (1) conventional needle looming and (2) the application of liquid forces against the batt to permanently entangle the fibers. Mechanical needling using a loom is well known to those skilled in the art. Hydraulic entangling methods generally include passing a layer of fibrous material on a suitable support under a plurality of individual high pressure liquid streams. Although the strength and coherency of the fabric web formed using this method may be satisfactory, it has been found that the individual liquid streams cause streaking or non-uniform fiber entanglement across the layer of fibrous material, i.e. in those areas of the material not contacted by the liquid streams. By the use of a hydraulic curtain in accordance with the preferred method of using this invention, this problem is avoided.
Devices are known which produce a curtain of liquid for the purpose of application against a layer of fibers in a slurry. However, these devices appear to be used for the rearrangement of the fibers in the batt and not their entanglement, in accordance with the objectives of this invention. For example, when the liquid curtain of the present invention is used to entangle the fibers in a batt into a coherent fabric web, the curtain is applied under a pressure sufficient to cause entanglement rather than just rearrangement of the fibers. The curtain is substantially non-diverging and uniform in width throughout its length, which results in uniform fiber entanglement. When the methods and devices of the prior art are utilized at a pressure sufficient to effect fiber entanglement, a substantially non-diverging, liquid curtain of a uniform width will not be obtained because no adequate means are provided for obtaining such a uniform width, non-divergent curtain.
Thus, the present invention may be utilized to avoid streaking or uneven fiber entanglement. Also, uniform entanglement may be achieved because a uniform width, substantially non-diverging curtain can be produced according to this invention under a pressure sufficient to effect fiber entanglement. Another advantage of the present invention is that by utilizing a slot formed between two members secured together, a uniform width non-divergent curtain of liquid is more readily obtained that if the slot is formed by a cut made through a single member. It has been found to be difficult to cut a very narrow slot through a single member so that the surfaces defining the slot are even. Practically, the machining of the slot cannot be accomplished to result in a very thin slot having smooth faces. By initially making individual members and then securing the members together as in the present invention, unevenness of the slot defining surfaces of the members is no problem.
In addition to the advantage of the device of the present invention that it is capable of producing a substantially non-diverging, uniform width liquid curtain, means for varying the width of the curtain are also provided. Thus, the same device may be utilized to produce curtains of different widths without the necessity for complicated restructuring of the device for this purpose or for storing of several devices that each produce a curtain of a specific width. Another advantage of the device of the present invention is that it produces a liquid curtain whose width and divergence are essentially independent of liquid pressures over a wide range. Another advantage of the device is that, if one desires, the device surfaces defining the slot can be machined to have a profile (other than being flat through their length) which provides various desired characteristics of the curtain to be obtained without increase of the divergence of the curtain.